Optical fiber transmission systems are being extensively used in the telephone network for long distance and interoffice trunk lines because of their wide bandwidth, small size and insensitivity to electrical interference. Conventional long distance optical transmission systems utilize time division multiplexed digital transmission. The maximum data rate available in commercial lightwave systems was for many years limited to 565 megabits per second, and has only recently been increased to 1.7 gigabits per second. A 565 megabits per second optical trunk line carrying 8000 voice channels is very cost effective for voice transmission.
Recently, efforts have been made in the telecommunications industry to utilize optical transmission systems in the local, or subscriber, loop between the central office and individual subscribers. The goal is to provide not only voice, but also data and video transmission over the optical fiber to every home and business. The video services are expected to include not only broadcast services but also switched video services which will enable each subscriber to select programming and movies from video libraries. An uncompressed digital video signal requires a data rate of about 100 megabits per second, and analog FM video requires a bandwidth of about 30 MHz. The 565 megabits per second system can carry only a few video channels.
Increased transmission bandwidth can be realized with coherent optical systems using multiple modulated optical carriers which are closely spaced in frequency. Coherent systems with multiple optical carriers have been disclosed by Shikada in "Multiplex Transmitting Method for Optical Heterodyne/Homodyne Detection Wavelength", Japanese patent publication No. 62-43231, 1987. In the Shikada system, one information channel is transmitted on each optical carrier, but N optical carriers can be utilized.
Subcarrier multiplexed (SCM) optical communication systems have also been proposed as a means for providing increased transmission bandwidth. A wideband signal composed of many frequency multiplexed carriers at either RF or microwave frequencies is used to modulate an optical carrier. The optical signal is transmitted through a conventional single mode optical fiber to a remote location. The optical signal received at the remote location is detected with a high speed photodiode, and the transmitted signals are recovered with a conventional RF or microwave receiver. The RF or microwave carriers can be modulated by either analog or digital signals and can be used to carry voice, data, video, digital audio and high definition video, in almost any combination of services.
Transmission of 60 frequency modulated video channels over 18 kilometers of optical fiber is described by R. Olshansky et al in "60-Channel FM Video Subcarrier Multiplexed Optical Communication System", Electronics Letters, Vol. 23, No. 22, pages 1196-1198, October 1987. A coherent subcarrier multiplexed optical communication system is disclosed in pending application Ser. No. 07/289,009 filed Dec. 22, 1988. In the disclosed coherent SCM system, M information channels can be transmitted on an optical carrier by using M subcarriers, each individually modulated with a separate information signal. By combining the above techniques, M information channels can be carried on each of N optical carriers for a total of M.times.N information channels.
A drawback of an SCM system with multiple optical carriers is that each modulated optical carrier has upper and lower sidebands and also contains second order intermodulation products which fall outside the upper and lower sidebands. To prevent interference between adjacent optical channels, it is necessary to provide a frequency separation between optical carriers of 3 f.sub.max, where f.sub.max is the maximum subcarrier frequency used to modulate the optical carrier. In order to maximize the transmission bandwidth, it is desirable to achieve close spacing between optical carriers, while minimizing interference.
Quadrature optical modulation involves modulation of two coherent optical carrier components that are 90.degree. apart in phase by separate modulating signals. In quadrature phase sift keying, the modulating signals are digital. Quadrature modulation can be used to simultaneously transmit two independent information signals with a single optical carrier frequency.
It is a general object of the present invention to provide improved optical communication systems.
It is another object of the present invention to provide optical communication systems having a very large information-carrying capability.
It is a further object of the present invention to provide a quadrature phase shift keyed optical modulator.
It is another object of the present invention to provide a quadrature optical phase modulator for subcarrier multiplexed optical communication systems.
It is yet another object of the present invention to provide a quadrature optical phase modulator wherein second-order intermodulation products are cancelled.